An intake system of this type is provided with havings a bypass passage with havings a control valve for allowing a portion of intake air to flow in an intake pipe, bypassing a throttle valve disposed in the intake pipe. The control valve is feedback controlled so that a desired speed of rotation is reached both when an engine is idling while the throttle valve is fully closed and when it is correctively controlled according to atmospheric pressure. That is, when running at high elevations, where atmospheric pressure is low, because intake air pressure or boost pressure in the intake pipe downstream of the throttle valve is low and that the density of air is low, the air absorbing force in the bypass passage is insufficient. For this reason, a control value, by which the control valve of the bypass passage is opened, is increasingly corrected, so as to provide a sufficient quantity of air distributed into the bypass passage. Thereby, a reliable feedback control of engine idling speed is provided. Such an intake system is known from Japanese Unexamined Patent Publication No. 57(1982)-131841, entitled Idle Speed Control System For Internal Combustion Engine, published Sept. 14, 1982.
Transmission cooperating with engines are divided into two types, namely, manual transmissions and automatic transmissions. Because an engine with an automatic transmission is subjected to a larger load than an engine with a manual transmission, boost pressure in the engine with an automatic transmission is closer to atmospheric pressure as compared to an engine with a manual transmission and, accordingly, the difference between atmospheric pressure and boost pressure in the engine with an automatic transmission becomes smaller than in the engine with a manual transmission. A decrease of the pressure difference, which is slight at low elevations where atmospheric pressure is high, becomes significant at high elevations where atmospheric pressure is low. If the intake system cooperating with the automobile engine is operated with an automatic transmission in which air or an air mixture such as purge air is supplementarily supplied into the intake pipe downstream of the throttle valve in the same manner as the intake system cooperating with the automobile engine with a manual transmission, the pressure difference becomes too small at high elevations to provide a sufficient suction force or to assure that a sufficient quantity of supplementary air is supplied into the intake pipe.